Field of the Invention
Embodiments of the present invention relate, in general, to cognitive radio networks and more particularly to on-demand spectrum contention protocol specifications.
Relevant Background
Modern society is increasingly dependent on the radio spectrum. The rapid increase in wireless services and devices such as mobile communications, public safety, Wi-Fi, and informational broadcast serve as indisputable examples of how society uses the radio spectrum on a day-to-day basis. Unlicensed transmission bands can play a key role in the wireless ecosystem. Specifically Television (“TV”) bands are significantly underutilized.
Cognitive Radio (“CR”) is an enabling technology that allows unlicensed radio transmitters to operate in the licensed bands at locations when that spectrum is temporarily not in use. Based on cognitive radio technology the Institute of Electrical and Electronics Engineers (“IEEE”), following a Federal Communication Commission (“FCC”) Notice of Proposed Rulemaking in 2004, has fostered 802.22 as an emerging standard for Wireless Regional Area Networks (“WRAN”) aiming to provide alternative broadband wireless access in, among other places, rural areas. CR operates on a license-exempt and non-interference basis in the TV band (between 47-910 MHz) without creating harmful interference to the licensed services, which include, among others, Digital TV (“DTV”) and Part 74 devices (e.g. wireless microphones).
In a typical deployment scenario, multiple WRAN cells, each of which comprises a base station (“BS”) and associated customer premise equipments (“CPE”), may operate in the same vicinity while coexisting with DTV and Part 74 devices. In order to effectively avoid harmful interference to these licensed incumbents, the set of channels on which the WRAN cells are allowed to operate could be quite limited. For example as shown in FIG. 1, residing within the protection contours of DTV 140 and wireless microphones 150, both Network 1 110 and Network 3 130 are only allowed to operate on channel A, while Network 2 1220 may occupy either channel A or B, assuming that in total only 3 channels (channels A, B and C) are available. If WRAN1 and WRAN3 (also referred to herein as Network 1 and Network 2) attempt to perform data transmissions on channel A simultaneously, mutual interference between these collocated WRAN cells could degrade the system performance significantly.
The coexistence (sharing of resources) between incumbent users and secondary users is referred to as incumbent coexistence, and the coexistence between WRAN cells is referred to as self-coexistence. There are two main objectives in self-coexistence: minimizing the self-interference between co-channel overlapping cells, and satisfying the Quality-of-Signal (“QoS”) of the cells' admitted service workloads in a dynamic spectrum excess environment.
Distributed, cooperative, and real-time spectrum resource sharing protocol is called On-Demand Spectrum Contention (“ODSC”). The basic mechanism of ODSC is as follows: on an on-demand basis, BSs of the coexisting WRAN cells contend for the shared spectrum by exchanging and comparing randomly generated spectrum access priority numbers via Medium Access Control (“MAC”) layer messaging on an independently accessible inter-network communication channel. The contention decisions are made by the coexisting network cells in a distributed way. Only the winner cell, which possesses a higher spectrum access priority compared to those of the other contending cells (the losers), can occupy the shared spectrum.
The effectiveness of the ODSC protocol relies on the availability of an efficient and reliable inter-network communication channel for the interactive MAC message exchanges among network cells. In fact, in addition to supporting cooperative spectrum sharing protocols such as ODSC, a reliable inter-network communication channel is also indispensable to other inter-network coordinated functions for 802.22 WRAN and, in general, other types of cognitive radio based networks (e.g. inter-network synchronization of quiet periods for spectrum sensing, and coordinated frequency hopping). Beacon-based inter-network communication protocol called Beacon Period Framing (“BPF”) protocol is another technique used to realize a means for reliable, efficient, and scalable inter-network communication channel sharing by reusing the Radio Frequency (“RF”) channels occupied by the network cells.
ODSC is a coexistence protocol that employs interactive MAC messaging on the inter-network communication channel to provide efficient, scalable, and fair inter-network spectrum sharing among the coexisting WRAN cells. To achieve these design goals, ODSC allows the coexisting WRAN cells to compete for the shared spectrum by exchanging and comparing randomly generated contention access priority numbers carried in the MAC messages. Such spectrum contention process is iteratively driven by spectrum contention demands (i.e. intra-cell demands for additional spectrum resources to support data services, and inter-cell demands requesting for spectrum acquisitions). The contention decisions are made by the coexisting network cells in a distributed way, which allows an arbitrary number of cells to contend for the shared spectrum in the proximities without relying on a central arbiter. Instead of behaving selfishly, the competing cells cooperate with one another to achieve the goals of fair spectrum sharing and efficient spectrum utilization.
Currently, before initiating MAC layer messaging of the ODSC protocol, a WRAN cell that is demanding additional spectrum resource first evaluates and selects a channel on which no incumbent is detected. The cell then verifies whether the selected channel can be shared, employing the transmit power control (“TPC”) technique, with all other co-channel communication systems in the neighborhood. If it is feasible, the WRAN cell schedules its data transmissions on the selected channels with appropriate TPC settings. Otherwise, ODSC messaging takes place allowing cooperative spectrum contention among WRAN cells to share the target channel in a time-sharing manner.
As can be appreciated by one skilled in the relevant art, overlapping (one-hop) cells must compete for the use of the same spectrum in order to minimize or eliminate mutual interference that may render both cells unreliable. As described in commonly assigned U.S. patent application Ser. No. 12/354,606 entitled, “On-Demand Spectrum Contention for Inter-Cell Spectrum Sharing in Cognitive Radio Networks”, upon capturing the use of a particular channel, the occupying WRAN cell, referred to as the ODSC destination (“DST”), announces the occupancy to other cells within one-hop using an ODSC announcement message (“ODSC_ANN”). Other spectrum-demanding WRAN cells, referred to individually as ODSC source (“SRC”), receives the ODSC announcement messages that are regularly broadcasted by the DST cells. If a SRC receives ODSC_ANN messages from multiple DSTs, it randomly selects one of the DSTs. Thereafter the SRC decodes the message from the selected DST and then sends an ODSC request message (“ODSC_REQ”) that it is seeking access to the channel occupied by the selected DST. The request message includes a spectrum access priority number (“SAPN”), which is either a floating point number uniformly selected from [0, 1] or a fixed point number uniformly selected from [0, 2x−1] (wherein x is the number of binary bits representing the fixed point number). Each DST maintains an ODSC_REQ window so as to allow multiple SRCs to submit ODSC_REQ messages at different time instances without losing its own fair chance to participate in the contention process. At the end of an ODSC_REQ window, if any ODSC_REQs are received, the DST randomly generates its own SAPN and compares it with the smallest SAPN carried in the received ODSC_REQ messages. When the DST's SAPN is smaller (i.e. possesses higher priority), DST sends each SRC an ODSC response message (“ODSC_RSP”) indicating a contention failure. Otherwise, the SRC with the smallest SAPN will receive an ODSC_RSP with an indication of contention success meaning that access and control of the spectrum resource (channel) will be relinquished by the DST in favor of the winning SRC. The DST also sends a message to the other SRCs informing the SRCs of a contention failure. As one skilled in the art will recognize other criteria may be used to determine SAPN priority. For example, the contention participant possessing the largest SAPN may win the contention in another embodiment of the invention.
Upon receiving a success notice, the winner SRC broadcasts an ODSC acknowledgement (“ODSC_ACK”) indicating the time, Tacq, at which it intends to acquire the channel from the selected DST. All DSTs that are on the same channel as the one being contended for and are within a one-hop distance of the winner SRC respond to the ODSC_ACK by scheduling a channel release to occur at Tacq and broadcast an ODSC release message (“ODSC_REL”) to the neighborhood. The ODSC_REL contains information about the channel to release, the channel release time (set to Tacq), and the identification of the winner SRC that will acquire the channel. If the ODSC_ACKs are received from multiple SRCs before the channel is released, a DST selects the earliest Tacq specified in the received ODSC_ACK as the channel release time. This avoids collisions between the neighboring DST and SRC when the channel switching times do not agree. All SRCs that capture the ODSC_REL will also schedule channel acquisitions at Tacq as long as it is determined from the ODSC_REL that the one-hop DST is releasing the channel to either itself or to a winner SRC that is multiple hops away. On the other hand, if multiple ODSC_RELs with different Tacq are received before the channel switching, the earliest Tacq is taken for channel acquisition.
In a large scale network, it is likely that multiple DSTs and multiple SRCs coexist. As the contention processes are fully random and independent, different SRCs could select their own DSTs to contend for the same spectrum resource and the contention outcomes (i.e. winners of the contention and channel acquisition/release times) could be in conflict. The ODSC message flow described above is designed to coordinate the discrepancies between the conflicting contention decisions in order to ensure the stability of the coexistence behaviors and avoid loss of spectrum efficiency across the networks. However, at any one time only one network cell can utilize the shared channel in close proximity. While the network cell occupying the channel sends and receives data over a particular period of time, other neighboring network cells remain idle. This is true even when the network cell occupying the channel may not be fully utilizing the bandwidth of the channel over its allocated period of time. Additionally a network cell demanding a spectrum resource would have to remain idle for a relatively long duration (in the order of plurality of frames) until the channel to share is released by the occupying network cell. As a consequence, such a potentially long turnaround time of channel acquisition may negatively impact the quality of service (“QoS”) of time sensitive applications due to the long service interruptions. Spectrum sharing on a finer granularity than a channel (such as frame-based) is advantageous to enhance both the utilization of the operating spectrum and the QoS of the application.
What is needed is a set of general mechanisms for an arbitrary number of distributed network devices to share limited spectrum resources. Although the above description of the ODSC protocol outlines how a protocol is employed for resolving problems of radio resource sharing where the basic unit of the spectrum resource is a radio frequency channel, the same principal of ODSC applicability, without loss of generality, is desirable to other, more fine grained, apportionment of the shared spectrum. It is desirable to apportion the shared radio spectrum so that any effective combination of radio spectrum resource in both the time and frequency domain, such as a frame on a frequency channel or multiple frames on multiple frequency channels, can be effectively shared.
The present invention addresses a mechanism and special features of the ODSC protocol of spectrum sharing on a frame-by-frame basis. This protocol is referred to hereafter as frame-based, on-demand spectrum contention. These and other improvements to the prior art are addressed by one or more features of the present invention.